1. Field of the Invention
The present invention relates to metallized substrate, and more particularly, to a multiplicity of layers wherein each layer comprises a multiplicity of metallic nanoparticles having substantially the same size to provide surfaces that exhibit metal enhanced luminescence.
2. Related Art
In recent years it has been described and demonstrated that there are many new favourable photophysical effects of electronically excited states (fluorophores/luminophores) in close-proximity to plasmon resonant particles.(1-4) The coupled fluorophore lifetime is also observed to be much shorter than the “free-space lifetime,” reflecting the very fast “plasmon” lifetime of the coupled quanta. (5-7) Further, it is thought that the metal-enhanced fluorescence (MEF) effect is a consequence of an excited state coupling with the scattering mode of nanoparticles, which is thought to account for very fast MEF lifetimes, i.e. a coupled elastic scattering event.(3,8) For metallic nanoparticles, the extinction spectrum is comprised of both an absorption and scattering component.(9) For smaller particles (<25 nm), then the extinction is for the most part dominated by absorption, while for larger particles, by scattering.
While the mechanism for MEF is fairly new,(3) compelling data has been reported wherein: i) MEF is seen to be more pronounced for larger particles; ii) the wavelength dependence of MEF suggests a correlation between the scattering spectra of the nanoparticles and the emission spectra of the fluorophores; iii) MEF is angular dependent,(10) both from an observation and excitation perspective, similar to scattering by nanoparticles themselves(11); iv) metals ideal for MEF are those with high free electron densities(12) and v) metallic material has been shown to couple fluorophore emission, when fluorophores are positioned less than 100 nm from the surface. In addition, the coupled emission has been shown to be completely p-polarized, strongly indicating that the coupled-plasmon system is radiating.(13,14)
Silver is usually the noble metal of choice (12,15-17) for applications in MEF and Surface-Enhanced Raman Spectroscopy. The preparation of silvered substrates include several modes of deposition, such as by wet chemistry,(18) a layer-by-layer deposition technique,(19) electrochemically,(20) on glass,(21) and plastic substrates. (22) One of the most commonly used techniques is vapor-deposition as it yields the most reproducible substrates amongst the deposition techniques aforementioned.
However, heretofore producing silver substrates for providing the MEF effect has included a single layer of particles deposited directly on the substrate. Although the results have been found to be very satisfactory, it would be advantageous to provide a substrate with multiple layers of such nanoparticles thereby providing further enhancement of the signal and methods of forming same.